Electric motors of various types are commonly used to rapidly move an object linearly from one stationary position to another or to rotate an object from one stationary angular position to another by accelerating and then decelerating the object. The motor converts electric energy into the kinetic energy of the moving object during acceleration, and then brakes the object by converting that kinetic energy into another form such as heat during the deceleration. However, in some applications such motors are incapable of providing adequate speed of movement while also limiting their power consumption, size, and control complexity to acceptable levels. Such electric motors may serve as rotary or linear actuators and have been used in various applications such as, but not limited to, internal combustion engine valves.
Another method commonly used to rapidly move or rotate an object is to employ a mechanical spring. The initial position of the object is set by stretching or compressing a spring, or set of springs, such that a certain amount of potential energy is stored in the springs. When the spring is released the object is accelerated and the potential energy is converted into kinetic energy. After the object passes the position where all potential energy has been converted, it begins to decelerate and the kinetic energy is converted back to potential energy. The object can be captured at the end of its travel with the same potential energy as it had at the start, at the cost of only a small input of energy to compensate for energy lost to friction and heat, for example. In comparison with motor-driven switching, a spring mechanism involves greatly reduced power consumption due to its ability to efficiently convert potential energy to kinetic energy, and vice-versa, in a passive manner. In many applications a spring mechanism is also simpler and smaller.
Conventional mechanical springs are shown in FIGS. 1a and 1c. FIG. 1a shows a conventional coil spring system 10. Assuming the left end of spring 16 is attached to stationary element 12, when moving element 14, attached to the right end of spring 16, is moved laterally, left or right, the spring produces a linear force as depicted in the graph of FIG. 1c. Specifically, when the spring is stretched to the right it produces a negative force, proportional to the offset or distance of movement, which tends to restore the spring to its original length. When the spring is compressed to the left it produces a positive force, proportional to the offset or distance of movement, which again tends to restore the spring to its original length.
FIG. 1b shows a known arrangement of magnets 20 that exhibits behavior analogous to that of a coil spring. Note that permanent magnets are depicted in FIG. 1b, but electromagnets can be conventionally substituted in any such arrangement to produce the same effect as permanent magnets. Examples of such magnetic springs may be found in the linear magnetic springs of U.S. Pat. No. 5,017,819 issued to Patt et al. and U.S. Pat. No. 5,148,066 issued to Beale et al. and the rotary magnetic spring of U.S. Pat. No. 5,038,063, each of which are herein incorporated by reference.
With continued reference to FIG. 1b, it is shown that if the leftmost magnet 22 and the rightmost magnet 26 are fixed in stationary positions and the center magnet 24 is moved left or right, within a limited range of movement the interaction of the magnetic fields of the three magnets produces a restoring force similar to that depicted in the graph of FIG. 1c. In FIG. 1d, a graph shows the preferred force characteristics versus offset of an idealized, hypothetical high performance switching mechanism. Mechanical spring-based mechanisms are unable to achieve this type of performance, whereas such high performance is more likely via magnet-based mechanisms.
A major drawback that limits the achievable switching speed and utility of a mechanical spring-based switch is the large force or torque typically required to hold the object in position prior to or after the switching movements. If switching speed requirements are too high to allow mechanical capture at the end of travel, an electric actuator such as an electromagnet is needed to provide the holding force or torque, producing a constant power drain when the object is stationary. The force or torque of a mechanical spring increases in proportion to the displacement, as depicted in FIG. 1c. As the required switching speed increases, the force capability of the spring must also increase, which increases the required holding force or torque, which increases the required current in the electric actuator, which can quickly lead to unacceptable levels of power consumption. The high force that accompanies large displacements also makes it exceedingly difficult to achieve “soft landing”—i.e., a low impact speed at the end of travel.
Although the holding force in a mechanical spring system can sometimes be reduced through a complicated combination of springs and other mechanisms, such as in a compound bow, the complexity of those devices limits the switching speed and also brings about issues of size, cost, reliability, and so forth. Another drawback of a mechanical spring system relates to the periodic deformation of the spring that induces high-frequency internal friction in the spring material. This not only causes energy loss, but also is a potential source of fatigue failure in high speed switching applications.
It is desirable to have a switching mechanism that eliminates the need for significant holding force or torque at the stationary positions as well as the fatigue and energy loss characteristics of a spring, while retaining the advantages of a spring mechanism. Preferably, it would possess a force-displacement characteristic as shown in FIG. 1d. Here, the curve AOB represents a switching process between two stationary positions, A and B, where the spring force/torque is zero so that no hold force/torque is required. Since these points are located at the maximum displacements in a switching cycle, they correspond to the maximum potential energy and thus unstable equilibrium. Therefore, if the object is displaced a short distance from point A in the positive direction it would encounter a force/torque in the same direction that would tend to drive the object further from point A. When moving from point A to point 0, the object would be accelerated as potential energy is converted into kinetic energy. From point 0 to point B, the object would be decelerated as kinetic energy is converted back into potential energy. Virtually no energy input would be needed to accomplish the switching process. The middle point 0 is a stable equilibrium position.